Analysis of Pump Shaft (GA-207) Failures from the Synthane Pilot Plant: Failure Analysis Report

摘要

The 3.18-cm (1.25-in.) diameter AISI Type 4140 steel shaft and the AISI Type 4037 steel capscrew of water pump GA-207 from the Synthane plant failed by complete separation after approximately 1.8 to 2.9 Ms (500 to 800 h) operation. The pump circulates high-temperature water between the venturi scrubber and the scrubber surge tank. A replacement capscrew, Type 4037 steel, also failed by shear after a shorter operating time (9 to 18 ks (2.5 to 5 h)). Both failures are associated with fatigue. The capscrews failed in a brittle manner due to cracks that initiated at the thread root. The capscrew manufacturer suspects that this component was improperly heat treated during production. The shaft failure occurred shortly after the capscrew failed. A high torsional load due to carbon-fine buildup is transmitted to the keyway that engages the impeller of the pump after the capscrew fails. The keyway fillet radius (0.021 cm) was smaller than recommended (0.067 cm) by normal design practice and acts as a stress riser adding to the existing torsional loads. The Type 4037 steel capscrew failed as a result of improper heat treatment, as was suspected by the capscrew manufacturer. The improper heat treatment, was verified by a measurement of the capscrew hardness, which was twice that specified for this steel. The fracture initiated at surface defects and propagated in a brittle manner as a result of high Vickers hardness. The Type 4140 steel shaft failure occurred after the capscrew fractured. A fatigue crack initiated at the keyway fillet radius of the shaft and propagated toward the shaft center and along the keyway. The small radius of curvature of the keyway fillet (0.021 cm) contributed a stress riser to the torsional load that led to the peeling-type crack. (ERA citation 04:021166)